Conventional spring loaded contact probes generally include a movable plunger and a barrel having an open end for containing an enlarged diameter section of the plunger, and a spring for biasing the travel of the plunger in the barrel. The plunger bearing slidably engages the inner surface of the barrel. The enlarged bearing section is retained in the barrel by a crimp near the barrel open end. The plunger is commonly biased outwardly, a selected distance by the spring and may be biased or depressed inwardly into the barrel, a selected distance, under force directed against the spring. Axial and side biasing of the plunger against the barrel prevents false opens or intermittent points of no contact between the plunger and the barrel. The plunger generally is solid and includes a head or tip for contacting electrical devices under test. The barrel may also include a tip opposite the barrel's open end.
The barrel, plunger and tips form an electrical interconnect between the electrical device under test and test equipment and as such, are manufactured from an electrically conductive material. Typically the probes are fitted into cavities formed through the thickness of a test plate or socket. Generally a contact side of the electrical device to be tested, such as an integrated circuit, is brought into pressure contact with the tips of the plungers protruding through one side of the test plate or test socket for manufacturing spring pressure against the electrical device. A contact plate connected to the test equipment is brought to contact with the tips of the plungers protruding from the other side of the test plate or test socket. The test equipment transmits signals to the contact plate from where they are transmitted through the test probe interconnects to the device being tested. After the electrical device has been tested, the pressure exerted by the spring probes is released and the device is removed from contact with the tip of each probe.
The process of making conventional spring probes involves separately producing the compression spring, the barrel and the plunger. The compression spring is wound and heat treated to produce a spring of a precise size and of a controlled spring force. The plunger is typically turned on a lathe and heat treated. The barrels are also sometimes heat treated. The barrels can be formed in a lathe or by a deep draw process. All components may be subjected to a plating process to enhance conductivity. The spring probe components are assembled either manually or by an automated process.
An important aspect of testing integrated circuits is that they are tested under high frequencies. As such impedance matching is required between the test equipment and the integrated circuit so as to avoid attenuation of the high frequency signals. Considering that spacing within a test socket is minimal, in order to avoid attenuation of the high frequency signals, the length of the electrical interconnect formed by the probes must be kept to a minimum. To address this problem external spring probes have been developed having a shorter length than conventional probes. External spring probes consist of two separate sections each having a tip and a flange. A contact component extends from each probe section opposite the tip. The two contact components contact each other and the spring is sandwiched between two flanges that surround the contact components. Typically the first contact component is a barrel while the second contact component is a bearing surface. The bearing surface is slidably engaged to the inner surface of the barrel. These probes are fitted into cavities formed in the test sockets used during testing. A problem associated with these type of external spring probes is the expense to manufacture due to costly machining operations.
In response thereto external spring probes were designed having flat components which can be produced less expensively by stamping. Typically these designs incorporate two components which are connected orthogonally and the electrical path between the two components is through a protruding end surface. A problem with this design is that the components wear out rather quickly and have a short life span requiring constant replacement.
Non-orthogonally connected external spring contact assemblies have two movable and linearly overlapping contact members or plungers surrounded by an external spring. Each plunger has a contact portion and a tail portion wherein the tail portion has a flat surface that passes over and makes contact with an opposing flat plunger tail portion inside the spring when assembled. The spring has end coils that press onto each of the opposing plungers to prevent the plungers from separating from the spring, thus fixing the plunger contact portion and the tail portions with respect to each end of the spring. Utilizing the natural torsional movement of the spring while it is compressed, the flat surfaces of the plunger tail portions maintain contact throughout the compression stroke of the contact assembly. The contact between the opposing flat sections prevents the twisting or torsional movement of the spring from translating to the tips on the contact portions. The opposition to the natural twisting enhances the electrical conductivity of the components, which in turn improves performance of the spring contact assembly. The spring can also have reduced diameter coil sections along the length of the spring to further constrain the plunger tails and enhance the interaction between the two plungers, or further biasing effect can be created by adding an offset coil section in the spring.
Each of the plungers formed in a generally cylindrical shape are by lathe, screw machine or other similar manufacturing equipment. Plungers formed in a generally flat shape are by stamping, etching, photolithography or other similar manufacturing technique for creating substantially two dimensional geometries.
A problem with obtaining a good test signal occurs when the test location is dirty or contaminated by excess solder, flux or oxides. When these contaminants are present a reliable connection between the test site and the contact tip of the test contact cannot be assured. Consequently, a need exists for compliant electrical test contacts having a contact tip which assures a connection with contaminated test locations.